1. Field of the Invention
This invention relates to refrigerants and, particularly, to mixtures suitable for use as substitutes for hydrochlorofluorocarbons (HCFCs) and chlorofluorocarbons (CFCs) in refrigeration. In particular the ternary compositions include components selected from fluorinated ethers, ethers and fluorinated hydrocarbons.
2. Description of Related Art
In recent years it has been suggested that release of certain hydrochlorofluorocarbons and chlorofluorocarbons may have adverse long-term effects on the earth""s atmosphere. Particularly, these compounds are thought to undergo a ultraviolet radiation induced decomposition in the upper atmosphere, releasing chlorine atoms which are thought to react with ozone. The reaction of chlorine with ozone could reduce the extent of the protective stratospheric ozone layer. Depletion of the ozone layer could permit increased penetration of harmful ultraviolet radiation through the upper reaches of the atmosphere, with deleterious effects on the biosphere. Although this theory is not universally accepted, there has been a growing movement of international scope toward control of the production and use of certain CFCs and HCFCs.
Particularly, government regulation is forcing a phasing out of certain chlorine-containing refrigerants, and replacement of them with environmentally safer compounds. Accordingly, considerable effort has been directed toward finding alternatives for CFCs and HCFCs. Suitable refrigerant substitutes would be expected to have both a lower potential to damage the atmosphere and an acceptably high performance in the particular refrigeration systems in which they are intended as replacements for the conventional or xe2x80x9cfirst-generationxe2x80x9d CFC or HCFC refrigerants. For example, Sherwood U.S. Pat. No. 5,713,211 describes certain fluoroethers said to be useful in secondary loop refrigeration.
Thus far, no single compound has yet proven completely satisfactory as a replacement for any of the conventional CFC and HCFC refrigerants. For example, HCFC 134a (see below) has proven to be a satisfactory substitute refrigerant for CFC-12 in most but not all applications. Much of the developmental effort in this field has been directed toward finding suitable mixtures, and particularly azeotropic or azeotrope-like mixtures. For example, Lunger et al. U.S. Pat. No. 5,670,079 describes non-flammable azeotropic and azeotrope-like mixtures of a hydrocarbon and a fluorine-containing molecule, in which the atmospheric boiling points of the components differ by at least 20xc2x0 C. Bivens et al. U.S. Pat. No. 4,810,403 describes refrigerant mixtures of three or more selected halocarbons, each one after the first having a successively higher boiling point, and having a temperature/pressure relation like that of dichlorodifluoro methane (CCl2F2, FC-12). Gage et al. U.S. Pat. No. 5,650,089 describes binary refrigerant mixtures, which may be azeotropic (or azeotrope-like) or zeotropic, in which the components are selected from certain hydrofluoroethers, hydrofluorocarbons, and hydrocarbons. Klug et al. U.S. Pat. No. 5,605,882 and U.S. Pat. No. 5,648,016 describe azeotropic and azeotrope-like mixtures of a fluoroether and a hydrofluorocarbon, said to be useful among other applications as refrigerants. Minor et al. U.S. Pat. No. 5,443,880 described binary refrigerant mixtures in which one of the components is a sulfur-containing compound. Pearson U.S. Pat. No. 5,108,637 describes refrigerants including ternary mixtures in xe2x80x9cnonflammable proportionsxe2x80x9d, which may or may not be azeotropic. Shiflett U.S. Pat. No. 5,709,092 describes nonflammable near-azeotropic or essentially constant boiling mixtures of at least two components, and particularly ternary mixtures of certain fluorocarbons, said to be especially useful as supermarket case refrigerants. All the patents and other publications cited in this application are hereby incorporated herein in their entirety.
Chlorodifluoromethane (CHClF2, R-22) is one refrigerant for which a suitable substitute has been especially difficult to find. Refrigerant-significant parameters of a refrigerant mixture suitable for use as a R-22 substitute include: low boiling point (Tb for R-22 is about xe2x88x9241xc2x0 C.); high critical temperature (Tc for R-22 is about 96xc2x0 C.); critical mixing temperature well below the boiling point; and an acceptable flammability limit. Preferably the refrigerant is substantially non-corrosive to parts and fittings of the refrigeration apparatus which it contacts in use.
The situation remains that improved replacement refrigerants having optimum properties have not been reported. The present invention provides improved replacement mixtures.
We have discovered that certain combinations of three or more different compounds can provide refrigerant mixtures having refrigerant-significant properties that approximate those of R-22 and, accordingly, these mixtures are highly suitable as a drop-in substitute in R-22 refrigeration applications. The mixtures include at least one fluoroether, which may be a perfluoroether, and one or more of a second fluoroether, an ether and a fluorinated hydrocarbon. Preferred R-22 substitutes according to the invention are near-azeotropic, as these do not fractionate significantly over the range of temperature in use. Accordingly, in one general aspect the invention features a refrigerant composition that is a mixture of at least three different components, in which at least a first one of the components is a fluorinated ether and each of the second and the third components is a fluorinated ether, an ether and a fluorinated hydrocarbon. In some embodiments, each of at least three components is present in a proportion of at least 10 mole percent, and more usually one or more (most usually all) of at least three components is present in a proportion of at least 20 mole percent. It is understood that the total mol % of the ternary mixtures described herein must equal 100%.
In some embodiments the fluorinated sulfur-containing compound, if optionally present, is a fluorinated methyl sulfide. Particular fluorinated methyl sulfides include bis(trifluoromethyl) sulfide, CF3SCF3 (116S); and trifluoromethylsulfur pentafluoride, CF3SF5 (18S).
In some embodiments the fluorinated hydrocarbon, where present, has the formula C(x)F(y)H(2x+2xe2x88x92y), where x=1 or 2, and 1xe2x89xa6yxe2x89xa62x. Particular fluorinated hydrocarbons include 1,1,1,2-tetrafluoroethane, CF3CH2F (HFC-134a); 1,1,1-trifluoroethane, CF3CH3 (HFC-143a); 1,1-difluoroethane, CH3CF2H (HFC-152a); fluoroethane, CH3CH2F (HFC-161); and difluoromethane, CH2F2 (HFC-32).
Embodiments of the invention that may provide particularly useful performance in R-22 refrigerant applications include mixtures of, for example: (218E/trifluoromethyl sulfur pentafluoride/152a) in molar proportions, e.g, (0.45/0.10/0.45); (218E/143aE/134a) in molar proportions, e.g., (0.35/0.35/0.3); (218E/143aE/161) in molar proportions, e.g., (0.333/0.333/0.333) or (0.40/0.40/0.20); (218E/134a/161) in molar proportions, e.g., (0.333/0.333/0.333); (125E/32/134a) in molar proportions (0.33/0.33/0.34); (125E/32/152a) in molar proportions, e.g., (0.33/0.33/0.34); (125E/143a/134a) in molar proportions (0.33/0.33/0.34).
In preferred embodiments the refrigerant composition is azeotropic or azeotrope-like. That is, it is a constant-boiling or substantially constant-boiling mixture of two or more substances that tends not to fractionate upon evaporation. Accordingly, the vapor produced by boiling or evaporation of the azeotropic or azeotrope-like mixture has the same or substantially the same composition as the liquid.
In another general aspect the invention features a method for cooling a body, by compressing a refrigeration composition according to the invention and bringing the body into heat transfer relation to it. The refrigeration compositions of the invention are effective drop-in substitutes for conventional CFC or HCFC refrigerants such as R-22, and can be employed in refrigeration apparatus designed for use with such conventional refrigerants, without significant retrofit or significant modifications. Some retrofit or modifications include, but are not limited to, compressor lubricant oils may need to be changed. Accordingly, in another general aspect the invention features a refrigeration system, including refrigeration apparatus configured and dimensioned for use with refrigerant R-22, charged with a refrigerant composition of the invention.
Definitions
As used herein:
xe2x80x9cFluorinated etherxe2x80x9d refers to the fluorinated ethers used in this art. Some are identified by a convention:
Two-component mixtures having as one component a very highly fluorinated ether and as another component an HFC or ether with a low fluorine content may have boiling points and critical temperatures approaching those of R-22, but these are usually not miscible over the range of temperatures and pressures they encounter in use. We have discovered that admixture of a third component, particularly an ether, having a fluorine content between that of the first two components, can provide for improved miscibility and better refrigerant-significant characteristics. Usually, the highly fluorinated ether component has more fluorine atoms than hydrogen atoms per molecule, and may in some embodiments be perfluorinated; and a molecule of the component having the lowest fluorine content has fewer fluorine atoms than hydrogen atoms.
Particularly, for use as R-22 substitutes, preferred mixtures have a boiling point lower than xe2x88x9230xc2x0 C., usually below about xe2x88x9232xc2x0 C., still more usually below about xe2x88x9233xc2x0 C. The boiling points are calculated from measured vapor pressure data using the following relation between Kelvin temperature (T) and the vapor pressure (P):
1n P=A+B 1n T+C/T,
in which A, B, and C are empirically derived constants.
Particularly for use as R-22 substitutes, preferred mixtures have a critical temperature higher than about 70xc2x0 C., still more usually higher than about 80xc2x0 C., and most usually in the range about 85-100xc2x0 C. The critical temperatures (Tc) of the mixtures are measured by slowly raising the temperature of the sample in a sealed tube until disappearance of the meniscus is observed. Alternatively or additionally, the critical temperature (Tc) can be estimated from the measured critical temperatures Tic, of the pure components and the component mole fractions Xic using the relation (for three components, i=1,2,3):
Tc=X1T1c+X2T2c+X3T3c.
Preferred mixtures have a critical mixing temperature below the boiling point, usually at least about 5xc2x0 C. below the boiling point.
The mixtures preferably are substantially non-flammable. Exceptions may include mixtures that contain both E-125 and R-161, and mixtures that contain both E-125 and R-152a. Generally, mixtures in which the fraction of Cxe2x80x94F and S-F bonds exceeds the fraction of Cxe2x80x94C and Cxe2x80x94H bonds are substantially non-flammable or have flammability limits that are acceptable for commercial units in the refrigeration industry.
The mixtures preferably are also substantially non-corrosive to fittings of standard R-22 refrigeration apparatus with which the refrigerant comes into contact in use.
Preferred mixtures are near-azeotropes; that is, they are characterized by showing less than about 2% separation at temperatures within the operational range.
The invention is now described in further detail.
Each of the components employed in the mixtures according to the invention is known, and either is commercially available in the desired purity or can be synthesized to the desired purity using published methods of synthesis. The patent literature in the refrigerant arts describes methods for synthesizing the various components, or refers to methods published elsewhere.
Particularly, where certain of the components may not be readily available commercially, the following methods are suggested.
218E (CAS Reg. No. 665-16-7) by direct fluorination of CF3OCH2F3 (prepared by reaction of CF3OF with vinyledine fluoride), see Sekiya et al., 1990, Chemistry Letters, pp. 767-70.
143aE (CAS Reg. No. 421-14-7) by reaction of methyl fluoroformate with sulfur tetrafluoride, see Aldrich et al., 1964, Jour. Am. Chem. Soc., Vol. 29, pp 11-15.
125E (CAS Reg. No. 3822-68-2) by electrochemical fluorination of dimethyl ether (CH3OCH3), see Fox et al. U.S. Pat. No. 511,760; Hutchinson U.S. Pat. No. 3,887,439.
116S (CAS Reg. No. 371-78-8) by thermal decomposition of CF3C(O)OSCF3, see Haas et al., 1969, Chem. Ber., Vol. 102, pp. 77-82.
SF5CF3, trifluoromethylsulfur pentafluoride (CAS Reg. No. 373-80-8) by reaction of methyl mercaptan with either cobalt trifluoride or fluorine gas, see Silvey et al. U.S. Pat. No. 2,697,726; Silvey et al., 1950, Jour. Am. Chem. Soc., Vol. 79, pp. 3624-26.
Compounds 32 [difluoromethane], 161 [fluoroethane], 152a [1,1-difluoroethane], 143a [1,1,1-trifluoroethane], 134a [1,1,1,2-tetrafluoroethane], 125 [pentafluoroethane] are commercially available and may where necessary be purified by distillation or other conventional method.
The components may be combined by any of a variety of conventional methods to yield a mixture having the desired proportions.
A refrigeration system may be charged with a refrigerant mixture according to the invention by conventional means. Because the mixtures are effective as drop-in substitutes or replacements for R-22, they may be used to charge any refrigeration apparatus designed for use with R-22 refrigerant without retrofitting or mechanical adaptation.